The smuggling of contraband, such as guns, explosives, drugs, as well as weapons of mass destruction, onto planes in carry-on bags and in luggage, as well as across borders and by boat in large cargo conveyances, such as cargo containers and pallets, is an ongoing concern.
Weapons of mass destruction that may be smuggled in cargo conveyances and smaller objects, include nuclear devices, such as atomic bombs and “dirty bombs,” which use a conventional explosion to disperse radioactive material over a wide territory. Radioactive, fissionable, fissile, and fertile materials that may be used to manufacture atomic devices, may also be smuggled in such objects. Fissile materials, such as uranium-235, uranium-233, and plutonium-239, may undergo fission by the capture of a slow (thermal) neutron. Fissionable materials include fissile materials, and materials that may undergo fission by capture of fast neutrons, such as uranium-238. Fertile materials may be converted into fissile materials by the capture of a slow (thermal) neutron. Uranium-238 and thorium-232, for example, may thereby be converted into plutonium-239 and uranium-233, respectively. Fissionable, fissile, and fertile material are referred to herein as “nuclear material.”
Radiation is commonly used in the non-invasive inspection of contents of objects, such as luggage, bags, briefcases, cargo containers, and the like, to identify hidden contraband at airports, seaports, and public buildings, for example. For example, in a line scanner, an object to be inspected is passed between a stationary source of radiation, such as X-ray radiation, and a stationary detector. The radiation is collimated into a fan beam or a pencil beam. Radiation transmitted through the object is attenuated to varying degrees by the contents. The attenuation of the radiation is a function of the density of the materials through which the radiation beam passes. The transmitted radiation is detected and measured. Radiographic images of the contents of the object may be generated for inspection. The images show the shape, size and varying densities of the contents.
The stationary source of radiation used in a common inspection system is typically a source of X-ray radiation of about 160 KeV to about 450 KeV. The X-ray source may be a source of Bremsstrahlung radiation, for example. The X-ray source in this energy range may be an X-ray tube. Objects may be scanned at more than one energy in the KeV range to obtain additional information concerning the material content of the object.
Standard cargo containers are typically 20-50 feet long (6.1-15.2 meters), 8 feet high (2.4 meters) and 6-9 feet wide (1.8-2.7 meters). Air cargo containers, which are used to contain a plurality of pieces of luggage or other cargo to be stored in the body of an airplane, may range in size (length, height, width) from about 35×21×21 inches (0.89×0.53×0.53 meters) up to about 240×118×96 inches (6.1×3.0×2.4 meters). Large collections of objects, such as many pieces of luggage, may also be supported on a pallet. Pallets, which may have supporting side walls, may be of comparable sizes as cargo containers and use of the term “cargo conveyance” encompasses cargo containers and pallets. X-ray radiation of 450 KeV will not completely penetrate large objects such as cargo containers. MeV (megavoltage) X-ray sources are required.
It has been found to be difficult to distinguish nuclear devices and nuclear materials from other dense items that may be contained within a cargo container by standard X-ray scanning. It has been suggested that additional information may be derived about the material composition of the contents of objects by X-ray scanning at multiple energies in the MeV range. For example, two X-ray beams with energy spectra may be provided by X-ray sources with accelerating potentials of 6 MV and 9 MV or higher, which generate X-ray radiation beams with peak energies of 6 MeV and 9 MeV, respectively. For an X-ray beam having a peak energy of 6 MeV, the X-ray radiation will be attenuated mainly by Compton scattering. There is not much pair production over most of that spectrum. For an X-ray beam having a peak energy of 9 MeV or higher, more pair production is induced. Compton scattering also takes place. A ratio of the transmitted radiation detected at two energy endpoints may be indicative of the atomic numbers of the material through which the radiation beam passes. Although pair production starts at 1.022 MeV, Compton scattering predominates until higher peak energies are reached. An example of such a process is described in U.S. Pat. No. 5,524,133, for example, where a ratio of the mean number of X-rays detected at each energy endpoint by the detector array as a whole for each slice or by the individual detectors of the array is determined and compared to a look up table to identify a mean atomic number corresponding to the ratio. The material content of the freight is thereby said to be determined.
However, as is known in the art, dual energy techniques that seek to determine the effective atomic numbers or identities of materials, are not effective in the MeV range. (See, for example, “Better Imaging: The Key to Better Cargo Inspection,” Moore, John F., et al., Port Technology International 2001, 10th ed., Vol. 4, 113-119; and “Processing of interlaced images in 4-10 MeV dual energy customs system for material recognition,” Ogarodnikov et al., Physical Review Special Tropics—Accelerators and Beams, Vol. 5, 104701-1-104701-11 (2002) (“Ogarodnikov”)). As described in more detail in the Background of U.S. Pat. No. 7,257,188 B2, a parent of the present application which is incorporated by reference herein, multiple measurements of X-ray attenuations through the same material have a statistical distribution with a high standard deviation. Therefore, determinations of effective atomic numbers and the actual identities of materials under examination by such measurements, in a reasonable period of time for commercial applications, suffer from high false positive rates. Ogarodnikov describes a dual energy technique, apparently using a ratio of effective absorption coefficients, averaged over a Bremsstrahlung spectrum, to generate a color image of a scanned cargo container. Different colors indicate that material is in one of up to four material groups—organic, organic-inorganic, inorganic, or heavy substances, using segmentation techniques.
The accuracy of a scanning system seeking to identify a material, such as uranium, for example, may be characterized by its “sensitivity” and its “specificity”. Sensitivity is the probability that the presence of uranium in a cargo conveyance will be identified. A system with high sensitivity will identify more true positives (correct identification of the presence of uranium) and fewer false negatives (missed detection of uranium) than a system with low sensitivity. However, increased sensitivity may result in an increase in the number of false positives, which may not be acceptable. Specificity, which is a statistical measure of accuracy, is the probability that the scanning system will properly identify the absence of uranium in a cargo conveyance, for example. A system with high specificity will identify fewer false positives (identification of uranium in a cargo conveyance when it is not present), than a system with low specificity.